Modern TDMA communication systems are known in a variety of configurations. FIG. 1A shows a typical point to multi-point communication system in which a receiver 102 (see also FIG. 3) communicates bi-directionally with a plurality of users 104-1 to 104-N. FIG. 1B shows data packets (in order of users 1, 2, 3 . . . N) arriving in order at the receiver. FIG. 2 shows schematically a particular, passive optical network (PON) communication system, in which the receiver is represented by an OLT 202 and the users are represented by ONUs 204-1 to 204-N. The PON may be of any known type, for example a Gigabit-capable Passive Optical Network (GPON) or Ethernet-capable Passive Optical Network (EPON)
A typical receiver 300 is shown in FIG. 3. The receiver can both transmit and receive. It includes an AFE (receiving analog front end) component 302, a CDR (receiving clock and data recovery) component 304 a logic unit (MAC) 306 and a transmitter TX 308, interconnected as shown. AFE 302 amplifies the low power data in the receive side. A first control signal (Reset AFE) 310 is supplied by the logic unit to assist the AFE in its functions, This Reset AFE signal assists the AFE to tune to the incoming data. The CDR locks to the input data. It acquires the frequency and the phase of an input data 312 signal and outputs a digital Received Data signal 314, synchronized with a Received Clock signal 315, to the logic unit. A second control signal (Reset CDR) 316 is supplied by the logic unit to assist the CDR in its functions. This Reset CDR signal assists the CDR to fast lock (frequency and phase) to the input data. The logic unit performs all the processing and controls the Ranging processes (explained below).
Ranging is performed in multi-user networks, in which the packet arrival time should be accurate and much smaller than the propagation delay. In general, in such networks, the Ranging targets include detection of a new user, synchronization of the receiver with the incoming bit stream of the new user; and estimation of the propagation delay (or round trip delay) of the new user. After the Ranging process is successfully finished, the arrival time of the new user's packet is known to the receiving system, and receiving from the new user can be accurately initiated by the receiver.
FIG. 4 shows a typical situation when a ranging window is allocated for a new user to join the network. A ranging window is a period of time in which all the new users can try and join to the network (in order not to collide with other users during Steady State operation, defined as the operation mode after successful completion of Ranging. The packets of different users may have different propagation delays (or round trip delays). In order to prevent contentions, the receiver must synchronize the different users. Before Ranging, the start of packet arrival time from the new user is unknown to the receiver. Although it is performed only when a new user joins the network, Ranging is a difficult task and needs to be carefully addressed. Various Ranging methods (or “processes”) are exemplarily described in U.S. Pat. Nos. 6,980,561, 6,948,184, 6,768,730, 6,215,792, 5,802,061, 7,251,240, 7,016,355, 6,853,624, 5,850,525, 5,379,299, 5,043,982 and 4,845,735, all incorporated herein by reference in their entirety.
FIG. 5 shows a common packet structure, as known in the art. A packet includes, in order, a Guard Time section 502, a Preamble section 504, a Delimiter section 506 and a Payload section 508. The Guard Time, Preamble and Delimiter sections are sometimes named “Header”. The Guard Time is a time period in which no energy is transmitted. The Preamble (or synchronization sequence) is a sequence of zeros and ones. The sequence should consist of a lot of transitions to help the CDR to lock. The Delimiter is a sequence of bits which must be different from the Preamble. Its main use is to assist in detecting the beginning of the Payload. The structure of the Header is defined to enable the clock recovery to lock on the right timing.
FIG. 6 shows a straightforward, prior art Ranging method, typical of those indicated in the abovementioned references. At the beginning of a ranging window, the receiver activates the AFE and starts looking for a Preamble and Delimiter of a Ranging Packet until it finds both. In the Ranging process, since the arrival time is not known, the Header is larger than in Steady State, in which the Header should be as small as possible. This is possible since the arrival time is known to the receiver. The straightforward Ranging process has no periodicity in it—it occurs once. The process is finalized (ends) when the Delimiter is found and the receiver starts receiving the data correctly. This straightforward method assumes that the AFE does not require special control signals synchronized with the incoming data, which is usually the case.
To summarize, this kind of Ranging is limited to only few cases and it can be used only:    1. when the AFE and the CDR can detect data without external control signals (usually with special timing requirements (the AFE and the Burst Mode CDR sometimes require that the control signals should be activated during the preamble reception)    2. when the AFE blocks the data when the signal is low (squelch). Otherwise, false alarm reception can mislead the Logic Unit. This requirement requires special hardware in the AFE, the CDR or both components.
The main drawback of this method appears when there is a chance of false alarms or when one of the above conditions is not fulfilled. If there is a false alarm, there is no second chance, the burst is not received and the Ranging needs to start again. Moreover, some types of receivers must be activated during the reception of data, so the above Ranging method can not work at all.
Usually, to enable the Ranging process, special hardware building blocks such as a Fast Power Detection flag, a Fast AGC (Automatic Gain Control), etc., are built in the front end of the system. These building blocks are expensive and suffer from limited performance in terms of probability of detection versus false alarms. They also require expensive calibration. Moreover, most communication receivers, for example Burst Mode optical receivers (optical receivers that are intended for burst operation) or Burst Mode CDRs require a reset signal to start a proper receiving operation. This reset is possible only after the Ranging process since the packet arrival time must be known to the receiver. Furthermore, in a noisy environment, the false alarm rate increases, which sometimes makes Ranging (using such hardware block) impossible.
Accordingly, there is a need for, and it would be advantageous to have a robust Ranging method, which requires no additional hardware for its implementation (except for the ability to receive data), and which can always guarantee successful Ranging.